Such a lighting device is known from U.S. Pat. No. 7,434,963 B2. Light of a first linear array of LEDs is focused by a collector lens on the one hand and by a curved mirror on the other, and is filtered by a filter that attenuates the light in a wavelength range with wavelengths of more than 550 nm. The light of the first linear array of LEDs is in a range from 575 nm to 450 nm, or from 525 nm to 425 nm, that means it is actually outside the sensitivity range of night vision devices, however any disturbing light in the cyan waveband is filtered out. Additionally, second and third linear arrays of LEDs are provided, whose light is filtered by the same filter but is focused in a different spatial angle. Further, it is described that the light emitting diodes have a band gap of at least 2 eV, and that the filter blocks electromagnetic radiation of a wavelength of more than 930 nm, or 900 nm, or 700 nm, respectively, or of a range of wavelengths between these valves and 1000 nm. U.S. Pat. No. 6,758,586 B1 relates to a lighting device that is compatible with the image reproduction system of night vision devices. The conditions under which this is possible are specified, namely; 1) the intensifier tubes should not entirely mask the pilot's visual field; and 2) the lighting equipment of the aircraft should not disturb the intensifier tubes of the night vision device by giving rise to disturbing halos or ghost images caused by reflection of illuminated objects on the windows of the cockpit. With regard to the configuration of night vision devices according to type I and type II, reference is made to the MIL-L-85762 A standard. Type I night vision devices are attached to the helmet of helicopter pilots with the screen in front of the pilot's eyes enabling him to see the control panel instruments when looking down. With type II night vision devices, the delivered images are projected on the control panel, so that the instruments and the night vision image can be viewed simultaneously. Furthermore, the risks of interference between the light sources of the aircraft and the night vision devices may further be avoided by providing the light sources in a monochromatic color that is as far away as possibly from the red wavelength band. Generation 2 and 3 type night vision devices do not have a passband limited to the infrared and have sensitivity in the red range to cover wavelengths between 600 and 770 nm. In practice, LEDs emitting green light centered at 555 nm, called the “aviation green”, are used for lighting purposes. However, a drawback thereof is that the colors of the control panel are distorted, thereby reducing readability. According to the teachings of U.S. Pat. No. 6,758,568 B1, the risk of interference between the lighting sources of an aircraft and the night vision devices can be surprisingly avoided by using white light in the lighting system which is emitted from a light source working by electroluminescence. Such white light derived from diodes does not activate the automatic gain control circuit of a night vision device, and according to the teachings of U.S. Pat. No. 6,758,568 B1 it does not even need to be filtered in the critical band of red. In practice, such light sources are not found. Many light sources emit far less than 1% of red light of a wavelength of greater than 650 nm. In fact this is very little, but due to the high gain of image intensifiers even an imperceptible glow of dark red light is unacceptable. The imperceptible glow of an image intensifier are described in the MIL-STD-3009 standard, for example.
In practice, however, the user might desire to use LEDs that emit critical light in the red range above 620 nm.
DE 42 19 740 B4 discloses a display device which comprises a display including LEDs and a filter arrangement including a dichroic filter, an infrared stop filer, and a circularly polarized filter, for use in conjunction with a night vision device. The LEDs emit green light from about 510 nm to 720 nm, with an emission peak at 570 nm. The filter arrangement attenuates the radiation in a range between about 620 nm and 720 nm.
From DE 10 2012 001 151 A1 a lighting device is known which produces mixed light using a white LED, a green LED, and a red LED, which mixed light is filtered by an optical filter means to block light of a cutoff wavelength greater than 610 nm. For this purpose, a so-called NVIS film or an interference filter means is used.
US standard MIL-STD-3009 includes qualitative specifications on the spectral sensitivity of different classes of night vision devices. From the requirement profile follows that edge filters which are available as interference filters should be used to block a certain red part of the spectrum of the illumination light.
The technical description of the color of a light is internationally standardized according to the CIE standard (see CIE web site). For NVIS compatible light sources, the CIE Universal Color Standard (UCS) of 1976 has been adopted in which the color location is specified by coordinates u′ and v′. An illustration of the CIE-LUV color space system can be found in Wikipedia where the normalized values u′ and v′ are explained, which define a chromaticity diagram according to CIE 1976 UCS.
One requirement on any lighting device is that the light has the desired spectral color. In conjunction with night vision devices, the lighting device must additionally have a specific “NVIS radiance” which is defined as the ratio of near infrared radiation to visible radiation. In order to satisfy both requirements, a near infrared absorbing filter is used which additionally absorbs a certain amount of long-wavelength red light. For ordinary light sources, such as incandescent lamps and white LEDs, this means a considerable spectral color change of the light source. For incandescent lamps whose light can be described by the color temperature it is rather easy to determine a suitable filter that makes the light source compatible with night vision devices. However, this does not apply to LEDs. LEDs of the same color temperature do not necessarily have the same color coordinates in the u′v′ diagram of the CIE-LUV color space system.